In most commercial and military aircraft, air is bled by a bleed air system from two or more of the aircraft's gas turbine engines to supply pneumatic and thermal power to a number of aircraft and engine systems; for example: an air conditioning system, a cabin pressurization system, a thermal anti-ice (TAI) system, and a thrust reverser system.
In conventional bleed air systems, each engine supplies bleed air to a common manifold. Air flow from each engine to the manifold is controlled by the bleed air system to provide compressed air of a certain pressure and temperature. To ensure that each of the engines is supplying near equal air flows into the manifold, flow balancing is typically employed.
U.S. Pat. No. 4,671,318 entitled, "Aircraft Engine Bleed Air Flow Balancing Technique" to Benson discloses a balanced flow bleed air control system, and thoroughly describes the problems associated with unequal bleed air flows. The '318 patent teaches sensing the pressure drops across the heat exchangers (i.e., the precoolers) associated with each of the engines, and using that information to adjust the area of each pressure regulating valve (PRV). However, the variation in the pressure drop across each heat exchanger resulting from contamination during use, manufacturing tolerances and other error sources, precludes this approach from meeting the flow sharing accuracy requirements for the latest generation of bleed air systems. That is, the mass flow of bleed air from each engine cannot be calculated to within the accuracies required to sufficiently balance the bleed air flows due to the errors associated with sensing the pressure drop across the heat exchangers.